Shaoshun Li

Shikonin and its derivatives: a patent review

Introduction: Shikonin and its derivatives are the main components of red pigment extracts from Lithospermum erythrorhizon, whose medicinal properties have been confirmed for a long history, and have aroused great interest as the hallmark molecules responsible for their significant biological activities, especially for their striking anticancer effects. Areas covered: Areas covered in this paper include a review of the total synthesis, biological effects and mechanisms of shikonin and its derivatives for their anticancer activities in the past decade, basing on literature and patents. The current state and problems are also discussed. Expert opinion: At present, screening for anticancer shikonin derivatives is based on cellular level to find compounds with stronger cytotoxicity. Though several compounds have been discovered with striking cytotoxicity in vitro, however, no selectivity was observed and undoubtedly, the further outcomes have been disappointing because of their great damage to normal cells. Meanwhile, the presumed mechanisms of action are also established in terms of their cytotoxicity. From a pharmacological point of view, most of the shikonin derivatives are at an early stage of their development, and thus it is difficult to determine the exact effectiveness in cancer treatment. With research in this field going deeper, it can be expected that, despite the difficulties, shikonin derivatives as potential anticancer agents will soon follow.

Synthesis and antitumour activity of β-hydroxyisovalerylshikonin analogues.

A series of novel β-hydroxyisovalerylshikonin analogues bearing oxygen-containing substituents at the side-chain hydroxyl of shikonin were designed and synthesized. The cytotoxicities of these compounds were evaluated in vitro against multi-drug resistant (MDR) cell lines DU-145 and HeLa. Most compounds exhibited significant inhibitory activity on both cell lines. The structure-activity relationship showed the analogues with ether substituents displayed the most potent antitumour activity and selective cytotoxicity towards DU-145. Among the compounds with ether substituents, increasing the steric hindrance in the carbon bearing β-hydroxyl or replace the β-hydroxyl with acetoxy or methoxy would lead to the decline of cytotoxicity.

Semi-synthesis and anti-tumor activity of 5,8-O-dimethyl acylshikonin derivatives

A set of twenty-two 5,8-O-dimethyl acylshikonin derivatives were designed and synthesized starting from shikonin. The cell-based investigation demonstrated that these dimethylated derivatives were less active than or equally effective to shikonin. However, the selective cytotoxicities toward MCF-7 were found among these derivatives, together with no toxicity in the normal cell. Furthermore, compounds 3f, 3p, 3r were subjected to KM mice suffering from S-180 carcinoma subcutaneously, which possessed more potent than Fluorouracil, a typical anticancer drug used clinically. So we may conclude that the modification to the mother nucleus of shikonin via the methylation is an available approach to acquiring anti-tumor agents with higher selectivity and lower toxicity.

Synthesis and evaluation of novel alkannin and shikonin oxime derivatives as potent antitumor agents

A set of forty alkannin and shikonin oxime derivatives were firstly designed and synthesized. Their cytotoxicities against three kinds of tumor cells and a normal cell line were tested and compared with alkannin and shikonin. The cell-based investigation demonstrated that some oxime derivatives were more or comparatively effective to the lead compounds, especially their selective and excellent antitumor activities towards K562 cells with no toxicity in normal cells. We may conclude that oximate modification to the mother nucleus of alkannin and shikonin is an available approach to acquire potent antitumor agents.

Flavonoids and Naphthoflavonoids: Wider Roles in the Modulation of Cytochrome P450 Family 1 Enzymes.

The human cytochrome P450 family 1 enzymes consist of three members, CYP1A1, CYP1A2 and CYP1B1, which are predominantly involved in the phase I metabolism of xenobiotics. Because they have been implicated in carcinogenesis, cancer progression, and drug resistance, the inhibition of these enzymes has been widely considered an effective oncological therapeutic strategy. Some natural and synthetic flavonoids and naphthoflavonoids have been extensively documented to exert pronounced influence in the modulation of CYP1s, including functioning as inhibitors, substrates, and aryl hydrocarbon receptor (AhR) ligands. However, the molecular determinants behind these effects are still unknown. This review summarizes the structural features responsible for the CYP1 inhibitory effects of the reported flavonoids and naphthoflavonoids. Additionally, a three‐dimensional quantitative structure–activity relationship (3D‐QSAR) study was performed to better understand the effect of their structural properties on biological activities. We hope this review provides a useful foundation for the rational design of potent and selective CYP1 isozyme inhibitors, thereby accelerating the drug discovery process.

Kinesin spindle protein (KSP) inhibitors in combination with chemotherapeutic agents for cancer therapy.

A diverse group of proteins, the activities of which are precisely orchestrated during mitosis, have emerged as targets for cancer therapeutics; these include the Aurora kinases (AKs), Polo‐like kinases (PLKs), and the kinesin spindle protein (KSP). KSP is essential for the proper separation of spindle poles during mitosis. Agents that target KSP selectively act on cells undergoing cell division, which means that KSP inhibitors are mitosis‐specific drugs, and have demonstrated remarkable activities in vitro. However, a significant obstacle to the success of KSP inhibitors is that these compounds, with tremendous efficacy in vitro, have demonstrated little or even no antitumor activity in vivo. Accumulated data suggest that a combination of KSP inhibitors with various cytostatic drugs will result in a more powerful tumor‐killing effect than monotherapy. Combination therapies might predominate and represent the next frontier in the discovery research of KSP inhibitors as potential anticancer drugs. Few published studies have reviewed combination therapy using KSP inhibitors. Herein we provide a comprehensive review of the literature on KSP inhibitor monotherapy and therapeutic combinations. The current state and problems are also discussed.

Design, Synthesis, and Biological Evaluation of Shikonin and Alkannin Derivatives as Potential Anticancer Agents via a Prodrug Approach

To minimize the cytotoxicity of shikonin and alkannin that arises through the generation of reactive oxygen species (ROS) and alkylation of the naphthazarin ring, two series of novel core‐scaffold‐modified shikonin and alkannin derivatives were designed. These derivatives, which differ in their configurational and positional isomerism (R‐, S‐, and 2‐ and 6‐isomers) were synthesized in high enantiomeric excess (>99 % ee). The selectivity of the dimethylated derivatives was significantly higher than the parent shikonin in vitro, but some side effects were still observed in vivo. Surprisingly, the dimethylated diacetyl derivatives with poor anticancer activity in vitro showed tumor‐inhibiting effects similar to paclitaxel without any toxicity in vivo. The anticancer activity of these derivatives is in agreement with their low ROS generation and alkylating capacity, emphasizing their potential as prodrugs. This strategy provides means to address the nonspecific cytotoxicity of naphthazarin analogues toward normal cells.

Antitumor Activity of DMAKO-05, a Novel Shikonin Derivative, and Its Metabolism in Rat Liver Microsome

The antitumor activity of shikonin derivatives is largely dependent on the generation of superoxide radicals and the alkylation activity of their naphthoquinone moiety. However, our recent study showed that 1,4-dioxime-5,8-dimethoxynaphthalene (DMAKO-05), a novel shikonin derivative, displayed more potential antitumor activity and less toxicity compared to fluorouracil (5-FU) both in vitro and in vivo, even though the hydroxyl and carbonyl groups of its naphthoquinone structure were shielded. To understand the underlying mechanisms, we investigated the metabolism of DMAKO-05 in rat liver microsomes. The kinetic analysis indicated that DMAKO-05 underwent a biphasic metabolism in rat liver microsomes. The inhibition experiments showed that CYP1A and CYP3A were the major enzymes in the metabolism of DMAKO-05, along with partial contribution from CYP2A. In addition, the structures of eight DMAKO-05 metabolites, which were characterized by accurate mass and MS/MS fragmentograms, implied that DMAKO-05 was mainly metabolized through the oxygenation of its naphthoquinone nucleus and the hydrolysis of its side chain, instead of the oxidation of hydroxyimine to ketone. Therefore, DMAKO-05 should not be considered as a traditional naphthoquinone prodrug.

Advance in Anti-tumor Mechanisms of Shikonin, Alkannin and their Derivatives

Shikonin, alkannin and their derivatives, the main ingredient of Lithospermum erythrorhizon and Arnebia euchroma (Royle) Johnst native to Inner Mongolian and Northwest of China respectively, hold promising potentials for antitumor effects via multiple-target mechanisms. This review will emphasize the importance of their antitumor activity in apoptosis, necroptosis and immunogenic cell death, and expound the relationship of their antitumor activity and naphthoquinone scaffold that could generate ROS and alkylating agent. Meanwhile, the antitumor mechanisms of naturally-occurring shikonin, alkannin and their derivatives, which were divided into the direct interaction involved in alkylating agent, covalently binding the DNA and protein, as well as the indirect interaction mediated by ROS, nonspecifically influencing the mitochondria or multiple signal pathways, will be systematically summarized and discussed.

Design, Synthesis and Anticancer Activity of Shikonin and Alkannin Derivatives with Different Substituents on the Naphthazarin Scaffold

Based on the asymmetric reduction of alkannin ketone derivative 4 by diisopinocampheylchloroborane(DIP-Cl), a series of shikonin and alkannin derivatives was designed, synthesized and their anticancer activities against various kinds of cell lines were evaluated. The in vitro biological experiments demonstrated that compound S-10, a 5,8-O-dimethyl-1,4-dioxime alkannin derivative, not only displayed excellent antiproliferative activity against cancer cells, but also exhibited low toxicity towards normal cells. It could induce HCT-116 cell apoptosis, but had no impact on the cell cycle. The underlying anticancer mechanism of S-10 is most likely different from other shikonin and alkannin derivatives.